1. Field of the Invention
The present invention relates to a method and an apparatus for generating a control signal for semiconductor switches for controlling power delivered to a load by a polyphase AC power line, more particularly by using digital techniques.
It applies more particularly, but not exclusively, to the generation of the synchronization signal for the digital microprocessor control of an electronic speed variator for a DC motor using a controllable valve rectifier circuit, for example, a thyristor power bridge, of the Graetz circuit type, fed by a multi phase current.
2. Description of the Prior Art
It is known that in a three phase current variator of this kind, the rectifier circuit is usually formed of a bridge comprising three pairs of thyristors connected in the following way:
The two thyristors of each of the pairs are connected in series in the same direction.
The junction point between the two thyristors of the same pair is connected to a corresponding phase of the three phase network.
The two free ends of the thyristors of the same pair are connected respectively to the two power supply terminals of the motor.
This rectifier must further comprise a circuit for the successive switching on of thyristor pairs of the bridge in synchronism with the zero cross-overs of the compound voltages of the network, but with a certain phase shift .PHI..
Adjustment of the output voltage of this rectifier is then obtained by varying this phase shift .PHI..
It is clear that in order to control the switching of the different thyristor pairs, measures should be taken for accurately determining the times when the phase voltages of the network pass through zero while distinguishing them from each other and to determine if required, depending on the required voltage, the phase shift angle .PHI. with which, from this time, switching will be effected.
It should be noted in this connection that the idea of using the alternating voltage of the network for generating a synchronization signal which may serve for controlling the switching of a thristor bridge is not new. It is in fact easy to produce such a synchronization signal from detection of the time when the phase voltages of the network pass through zero (or even through a given value). However, the systems used are space consuming and expensive, because they necessarily require a three phase transformer with complex windings (direct winding and reverse winding).